The present invention pertains to drill bits, more specifically to drag-type drill bits, and even more specifically to the cutter devices or cutters which are mounted on the bodies of such bits. The bits may be of the full bore or corehead type.
A typical bit includes an integral bit body, typically comprised of one or more body members of either tungsten carbide matrix material or a suitable metal such as steel. A plurality of cutter devices is mounted on the bit body. Each such cutter device typically has a stud portion, which is mounted in a pocket in a bit body member and defines one end of the device, and a cutting formation generally adjacent the other end.
The cutting formation is located on what may be considered a leading side of the device, so that as the bit is rotated in its intended direction in use, this cutting formation engages and drills the earth formation. The opposite side of the device is considered the trailing side, and the device may further be considered to have a pair of lateral sides, generally opposite each other, and interconnecting the leading and trailing sides. The leading side may also be referred to as the forward side, and the trailing side referred to as the rear side.
In use, tremendous forces are exerted on the cutting devices, particularly in the forward-to-rear direction. In some cases, cutting devices have been broken by these forces.
Accordingly, it is desirable to maximize the strength of these devices. However, this goal must be balanced and coordinated with other objectives and/or limitations.
For example, the design of the bit body necessarily places limits on the maximum transverse dimensions of the stud portion of the device, in both the forward-to-rear and lateral directions. Furthermore, the devices are often arranged in rows extending generally radially across the working end face of the bit body, and the performance of the bit is affected by the number of devices which may be placed in a row, or in other words, the spacing of the cutting formations of the devices in a given row. In general, at least for some types of earth formations, it is desirable to place these cutting formations as close together as possible, i.e. to place as many devices as possible in a given row on the bit body. However, a limiting factor on this objective is that there be adequate thickness of bit body material left between each two adjacent cutter devices.
Another important consideration is that it should be possible to manufacture a number of such cutter devices to fairly accurate dimensions, i.e. within fairly close tolerances, but without undue expense in the manufacturing process.
In the past, the stud portions of typical cutter devices have been generally cylindrical, except for a small groove or keyway on the rear or trailing side, which cooperates with a small protrusion in the respective pocket of the bit body to properly index or orient the cutter device with respect to the bit body.
Such cylindrical stud portions have not satisfied the above-mentioned goals or objectives to the extent desirable. If the diameter of such a cylindrical stud portion were chosen to correspond to the maximum forward-to-rear dimension considered practical for a given bit design, it would be necessary either to leave less than optimum amounts of bit body material between the pockets for adjacent cutter devices, or to space such adjacent pockets and cutter devices apart by an amount greater than that which would permit the desired number of cutter devices per row on the bit body. On the other hand, if a smaller diameter were chosen, so that more cutter devices could be used in a row without unduly thinning or weakening the portions of the bit body between adjacent cutter devices, the devices may not be strong enough in the forward-to-rear direction, and may break off in use, as described above.
Another potential problem with such conventional cylindrical stud portions is that the aforementioned keyway could represent a weakening concavity in the cross section.